Implement "Export Cited" for CSL and BST styles

Refs https://github.com/JabRef/jabref/issues/11829, https://github.com/JabRef/jabref/issues/16380, https://github.com/JabRef/jabref/pull/16461, https://github.com/JabRef/jabref/issues/16352 JStyles currently has a "Export Cited" feature which goes through the legacy code path (`Codec52`). How it works is - it looks up the entries in the library as per citation keys in the reference marks of the documents, and also collects crossreff'ed entries, and exports them. It would be wonderful to have this feature for CSL and BST as well, as all the information needed is already available for them as well. In https://github.com/JabRef/jabref/pull/16461#pullrequestreview-4842552374 the ask was to disable the button for non-jstyles. When this is implemented, it should be enabled as well. Similarly, the disabled tooltip requirement for this as mentioned in https://github.com/JabRef/jabref/issues/16352#issuecomment-5184530626 would also no longer be valid.